I'd consider an oil analysis. It will be able to determine if the oil is indeed being operated past it's optimum temps.
On an air cooled engine (especially a turbocharged one which keeps turbocharger bearing temps under control with oil cooling) cooler oil is definitely better.
Another option may also be a cheapo electric temp sensor jerry rigged on the cooler for a flight.
Last edited by Typhoon650; 13th May 2013 at 23:40.